The present invention relates to an ionization cell for a mass spectrometer. In particular, the invention is applicable to mass spectrometers in which a heated electrical filament emits electrons. The invention also relates to a leak detector comprising the ionization cell.
In a mass spectrometer a gaseous sample is analyzed by bombarding the sample with a flux of electrons and then making the ionized particles thus obtained move so as to then differentiate them for example depending on their trajectory. The mass spectrometers of leak detectors thus measure and quantify a tracer gas, such as helium.
Mass spectrometers comprise for example an ionization cell containing an ionization cage and a heating electric filament that emits electrons. The molecules of the gas to be analyzed are bombarded by the electron beam and a substantial part of the molecules of the gas to be analyzed is converted into ionized particles. These ionized particles are then accelerated by an electric field. They then arrive in a zone containing a magnetic field, which has the property of altering the trajectories of the ionized particles as a function of their mass. The current of ionized particles of the tracer gas is proportional to the partial pressure of the gas in the apparatus, and its measurement allows the value of the flow rate of the detected leak to be known.
In order to make the operation of the mass spectrometer more reliable, certain ionization cells contain two filaments. A working first filament is powered to produce the electron beam and a backup second filament is intended to be powered in the event of failure of the working first filament.
However, it has been observed that the waiting time required for the backup second filament to become operational, so as to allow stable and reproducible measurement representative of the quantity of tracer gas, can prove to be excessively long (a wait of up to two hours may be necessary).